Wave form modifying circuits



Oct. 31, 1939. M GHGER 2,178,340

WAVE F ORM MODIFYING CIRCUIT Filed March 29, 1937 Eig.i I

. INVENTOR MAX GEIGER wgzm ATTORNEY Patented Oct. 31, 1939 UNITED STATES WAVE FORM MODIFYING cmcorrs Max Geiger, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic, m. b. H., Berlin, Germany, a corporation of Germany Application March 29, 1937, Serial No. 133,625

In Germany April 3, 1936 2 Claims.

My invention relates to electrical wave shape modifying circuits and more particularly to an arrangement whereby electrical impulses having undesired shapes may be modified into impulses having a steep wave front of a desired shape, and such an arrangement is especially adaptable for use in television work.

In television equipment, especially transmitters, the problem of generating electrical impulses having a steep wave front rises continuously, and such impulses for use with television equipment as at present known and used demand the production of impulses or waves having a shape as nearly rectangular as possible. This is especially important in so far as the generation and reception of synchronizing impulses is concerned.

In most instances, the impulses available are those which present a flank or side which is curved in form rather than rectangular both at the topand at the bottom of the curve. In

prior practice it has been customary for the purposes of modifying these curved wave forms to use a circuit arrangement wherein several tubes are joined co-operatively, these tubes having a grid bias voltage source or sources which are so set that the steep flank portion of the curved impulses falls within the straight portion of the operating characteristic of the tube. However, the requisite circuit arrangement of such a nature becomes appreciably complicated because of the necessity of providing a plurality of tubes which cut-off a part of the impulse, that is to say, use only the steep flank portion of the im- 35 pressed impulses and, again, such a circuit arrangement necessitates the use of a distinct grid biasing voltage source for each tube. In my invention, a circuitarrangement is disclosed of a kind which when impulses having a 40 rounded flank both at the top and bottom, for instance, are impressed onto a single tube, there will be formed in the output circuit of said tube a wave shape in which the bottom curved portion is eliminated without the necessity of pro- 5 viding a gridsource of energy for the tube. This grid bias. However, in such schemes the time constant of the condenser and grid leak was chosen very high in comparison with the frequency of the impressed impulses. 1

In my invention, however, there is in general provided a circuit arrangement of an unvaried time constant in which the grid leak resistance is of such value that during the time when current flows in the control grid, such a drop of potential results that the grid is so biased that the steep portion of the flank of the impressed wave falls within the straight part of the tube operating characteristic.

My invention will best be understood by reference to the drawingin which Fig. 1 is an explanatory curve,

Fig. 2 is an embodiment of my invention,

Fig. 3 is an explanatory equivalent circuit,

Figs. 4 and 5 are embodiments of my invention, and v Figs. 6 and 7 are-explanatory curves.

Referring to Fig. 1, there is shown two impulses of a general wave shape in which synchronizing impulses used in the television art usually arrive at the receiver or the type of impulses which are usually available for synchronizing purposes.

Referring to Fig. 2, there is shown an exemplified embodiment of my invention in which in is a vacuum tube, II a resistance connected in the plate circuit thereof, l2 a condenser, is the the grid leak. The assumption is made that the impressed impulses are derived from a source of low internal resistance and are impressed on the left-hand coat of the condenser.

The conditions surrounding such a circuit scheme, at the time grid current begins to flow, may be so conceived that between grid and filament of the tube a source of D. C. voltage is supposed to exist, the latter having an internal resistance equal to the resistance of the grid filament path, and that in the outer circuit of the said source of D. C. voltage the grid leak I3 is indicated. What thus results is the equivalent circuit scheme shown in Fig. 3 where M is the source of D. C. voltage, and IS the inner resistance of the grid filament path. The voltage of the D. C. source l4 becomes divided between resistances l3 and I5 so that the junction of these resistances where the grid is connected assumes a potential which, for a fixed time constant of the resistance capacity mesh l2, l3, may be adjusted by the size of the resistance l3. The time constant of 12, I3, in this scheme,as alreadypointed'out above, has been chosen as high as has heretofore been customary for the production of a negative grid voltage, in other words, so high that in the interval of time between two impulses the negative grid biasing voltage is practically preserved unvaried. However, if resistance E3 is chosen correspondingly low (and the condenser I2 correspondingly large to preserve the same time constant) it is possible to make conditions so that the lower end of the plate current grid voltage characteristic comes to fall roughly at a point marked AB, in Fig. l, with the result that the impulse arising in the plate current is started at a very steep angle, with an incidental elimination of the lower curvature of the avail able impulse. When in this shape, the impulse is already far more favorable for practical use than in the originally available shape.

Now, a form of impulse such as made available by a circuit scheme as shown in Fig. 2 may be still further improved for a number of actual purposes by inserting between condenser i2 and the control grid of the tube ill a resistance.

An exemplified embodiment of this scheme is illustrated in Fig. 4, where the newly added re sistance is indicated at it. Across the said resistance the grid current sets up a fall of potential so that from the very incipiency of grid cnrrent fiow the control grid potential stays nearly unaltered, while the upper fiat portion of the in pulse front is nearly eliminated. In other words, the plate current will then be rectangular to a higher or marked a degree than what is attainable in a circuit organization, Fig. 2. The resistance I6 may here be low in contrast to resistance I 3, though it must be high in comparison with the inner resistance of the grid-cathode path, in other words, high compared with the equivalent resistance l5, Fig. 3.

It has also been discovered experimentally that by paralleling a condenser to resistance it; it is possible to create plate potential impulses, i. e., variations of voltage at the plate of tube it which present a far steeper front than attainable in a circuit organization according to Fig. -l which, in addition, will exhibit a perfect horizontal limitation. The circuit organization which must be adopted to this end is of the sort as shown by way of example in Fig. 5, where the newly added condenser is indicated at ll.

An explanation why by the addition of condenser l l a very satisfactory plate potential curve is realizable cannot be suggested in a simple way. What must be kept in mind in this connection is that the grid filament path has both an inner ohmic resistance as well as a capacity between grid and filament. This goes to show that at the instant when flow of grid current sets in, there is present in series to the resistance capacity mesh I6, ll another similar mesh (grid filament capacitance and shunted thereto the grid filament resistance), while moreover the grid filament resistance, i. e., the time constant of one of the resistance capacity meshes, varies with growth of the grid current. Now, the circuit actions which arise when a D. C. source of potential in the sense of Voltage source M, Fig. 3, is connected with the series arrangement comprising two resistance capacity meshes is not quite simple. The rise of potential of the junction point of both resistance capacity meshes (which point in the circuit organization Fig. 5 corresponds to the control grid potential) may, as a matter of fact, be influenced to a large degree by changes of dimension. However, the situation becomes quite unintelligible when the time constant of the resistance capacity mesh is altered, that is, only altered in the course of the switch action, and this is quite the case in regard to the grid filament path. It is likely that so far as the form of the plate potential is concerned, also the distributed capacities, e., the self-capacitance of resistance I l and the capacitance between plate and grid or filament are playing a certain part.

. At any rate, it has been discovered experimentally that the shape of the plate potential, which, in a circuit scheme shown in Fig. 4, has invariably a shape as roughly indicated in Fig. 6, can be altered by a circuit organization as in Fig. 5 by increasing the size of the resistance i6, as indicated in Fig. 7 by the graph C, and by increasing the size of condenser H, for instance, in a way as indicated by the graph E. If the size of i6 and ll be chosen suitably, it is possible to secure a perfectly horizontal boundary line or contour for the impulse, in other words, a perfectly rectangular shape of impulse.

What I claim is:

1. A wave form modifying circuit comprising a thermionic tube having anode, cathode and at least one control electrode, a time constant circuit comprising a resistance-condenser combination having a time constant high with respect to the frequency of the wave to be modified connected in the cathode-control electrode circuit of said tube, said resistance and said condenser having such values that the bottom portion of said impulses render said tube non-conducting, and the steep portion of said impulses falls within the straight line portion of the anode currentgrid voltage characteristic, and means for impressing said impulses on said control grid circuit comprising a source having a low internal resistance.

2. A wave form modifying circuit comprising a thermionic tube having anode, cathode and at least one control electrode, a time constant circuit comprising a resistance-condenser combina- 1 tion having a time constant high with respect to the frequency of the wave to be modified connected in the cathode-control electrode circuit of said tube, said resistance and said condenser having such values that the bottom portion of said imr.- 

